The present invention relates to a polyphenylene sulfide composition having improved melt flow properties and particularly low generation of flashes when injection molded, as well as excellent strength at welded part (hereinafter referred to as "weld strength") and impact strength and also relates to a method producing the same.
Injection molded products made of a composition of polyphenylene sulfide (hereinafter referred to as "PPS") containing a reinforcing fiber or filler are excellent in heat resistance and chemical resistance and moreover, they have an advantage, which molded product made of other engineering plastics do not have, such as excellent electric insulation and dimentional stability. Accordingly, PPS is used in many industrial fields.
However, since molecular weight of conventional PPS having not treated after its polymerization reaction is low, namely, its melt viscosity is too low to be fabricated as it is, to apply such PPS for a practical use, so far it can not avoid to be cured with a treatment such as thermal treatment under existence of oxygen gas.
Usually, as such curing is performed with cross-linking, the molded products of such polymer is relatively poor in its toughness and even when some reinforcing material or filler is added to the polymer, its molded products are poor in their impact strength and in their weld strength. Accordingly, prompt actions to improve those defects are keenly requested.
As an example of such improvement, PPS, which is manufactured by the process described in U.S. Pat. No. 4,645,826, having a repeating unit of ##STR2## not less than 70 mol% of the total repeating units and a substantially linear structure (hereinafter referred to as "the linear PPS") is excellent in its toughness. Further, molded products of a composition made of the linear PPS have a much better impact strength and weld strength than molded products of a conventional PPS composition. This is the reason why the linear PPS draw the attention of many people to be suitable for the molded products which have many thin parts or the products which have a piece of metal in them. However, even when this linear PPS having an excellent toughness is fabricated, for example, injection-molded into precision molds, it is prone to generate much amount of flash on the molds.
The present inventors have already proposed in Japanese Patent Application No. 62-84,110/1987 a method to obtain a composition which has good anti-flash property by blending specific silane compound to the linear PPS, increasing the melt viscosity and further, raising the dependency of the viscosity upon its shear rate.
Further, as the arts to obtain a composition by blending conventional PPS with a silane compound, there are several disclosures, for examples, (1) Japanese Patent Application Laid-Open (KOKAI) No. 59-31,503 (1984) discloses an electronic part encapsulated with a composition comprising PPS and mercaptosilane, being excellent in water resistance and in electric properties under high temperature and high humidity, (2) U.S. Pat. No. 4,451,601 discloses a composition containing polyarylene sulfide and at least one organosilane, having improved anti-hydrolytic property and (3) Japanese Patent Application Laid-Open(KOKAI) No. 55-29,526 (1980) discloses a composition comprising 100 parts by weight of PPS, 0 to 30 parts by weight of fillers and 0.01 to 10 parts by weight of aminoalkoxysilane or its hydrolytic condensate and being excellent in water resistance, humidity resistance and in electric properties.
Furthermore, Japanese Patent Application Laid-Open (KOKAI) No. 61-285,255 discloses a composition of PPS containing a fibrous filler and/or another silicate filler and a silane compound having at least two unsaturated double bonds in its molecular and/or a non-hydrochloride silane compound having at least one unsaturated double bond and at least one amino group in its molecular, being excellent in its strength and electrical properties.
In these already known arts, the silane compound has been used to improve various properties of the composition, namely, (1) adhesion between PPS and an inorganic filler and (2) strength, humidity resistance and electric properties. Among these improvements, Japanese Patent Application Laid-Open (KOKAI) No. 55-29,526 (1980) discloses the increase of its melt flow, namely, the reduction of its melt viscosity, by adding aminoalkoxysilane to PPS and U.S. Pat. No. 4,451,601 also discloses a similar effect to reduce its melt viscosity by adding .gamma.-ureidopropyl-triethoxysilane to PPS. Such effect of silane compounds to reduce melt viscosity is just contrary to the object of the present invention.
As a result of the present inventors' extensive studies to manufacture a composition of the linear PPS being excellent in impact and weld strength and having improved anti-flash properties accompanied with high melt viscosity with an inexpensive way, they have found the following facts and based on these findings, completed the present invention:
(1) A composition obtained by treating the linear PPS with an aqueous solution of an acid or a salt of a strong acid and a weak base, blending an aminoalkoxysilane compound to the treated PPS and melt-kneading the blend at the temperature higher than melting point of the linear PPS, can improve its melt properties, such as, an increase of its melt viscosity and an increase of the dependency of the melt viscosity on its shear rate, no matter what polymerization degree, that is, what melt viscosity the PPS has. Further, the linear PPS without the treatments with said aqueous solution can not have such remarkable improvement of its melt property after blending an aminoalkoxysilane compound and melt kneading the blend.
(2) The composition, obtained by treating the linear PPS with the above aqueous solution, blending with an aminoalkoxysilane compound and melt-kneading, can have an remarkably improved impact strength compared with the linear PPS without above treatments or the linear PPS only treated with the aqueous solution. Further, this composition can also have a largely improved anti-flash property during its molding. However, there is a sign that a trace of ejector pin remains on its molded product and also a release of the product from the mold does not go smoothly.
(3) The composition obtained by treating the linear PPS with said aqueous solution, blending with an aminoalkoxysilane compound and a releasing agent and melt-kneading has somewhat less effect on an improvement of its melt property but still has an excellent anti-flash property. Further, release of its molded products from the mold proceed smoothly. On the other hand, a composition, for comparison, obtained by treating the linear PPS with said aqueous solution, blending a releasing agent (without an aminoalkoxysilane compound) and melt-kneading has an improved property of being released from the mold but has an adverse effect to generate much amount of flash making its melt viscosity lower.